X-Ray film with reduced print-through

ABSTRACT

Silver halide X-ray film with reduced print-through and improved sensitometric characteristics is provided by a film support having on both sides a low speed, fine grain emulsion overcoated with a high speed, preferably large grain emulsion, the ratio of the speed of the fast to the slow emulsion being from two to ten.

United States Patent van Stappen Dec. 2, 11975 1 1 X-RAY FILM WITH REDUCED PRINT-THROUGH [75] lnvcntor: Albert L. van Stappen. Rumson.

[73] Assigncc: E. 1. Du Pont de Nemours &

Company, Wilmington, Del.

{22] Filed: June 24. 1974 [211 App]. No: 482,570

[52] US. Cl. 96/451; 96/68; 96/82; 250/323 [51] Int. Cl. G036 1/92; G03c 1/46 [38] Field of Search 96/82, 68, 45.1

[56] References Cited UNITED STATES PATENTS 2,887 379 5/1959 Blake et a1 v. 96/82 3.050.391 8/1962 'lhompsozn ct all. 96/68 3,140,179 7/1964 Russell r t 1. 96/68 3.418.123 12/1968 Huugh v v .4 96/68 3.591382 7/1971 Millikan 96/68 Primal-y ExuminerR0nz1ld H. Smith 10 Claims, 1 Drawing Figure X-RAY FILM WITH REDUCED PRINT-THROUGH BACKGROUND OF THE INVENTION wide exposure latitude, improved image sharpness, and

increased photographic speed.

2. Description of the Prior Art:

In order to have adequate speed and thus to reduce patient exposure to dangerous X-rays, conventional medical X-ray film elements usually have relatively thick emulsion layers comprised of gelatino silver bromide with relatively large grain size. Coating such thick layers on a single side of a support is difficult. Additionally, since these elements are usually machine processed under so-called fast-access conditions, a single thick layer is undesirable. Conventionally, therefore, it has become the practice to coat the emulsion on both sides of the film support, one half on each side. Two X-ray intensifying screens are then employed. In theory, the X-radiation, passes through the emulsion and strikes the screen. Visible light is then emitted by the excitation process on the screen, causing exposure of the photographically sensitive emulsion. Each individual screen should expose only the emulsion layer nearest thereto. In fact, however, some light continues through the support and exposes the emulsion on the opposite side. This phenomenon, termed printthrough, can cause the resulting image to appear unsharp, which is an undesirable feature and cannot be tolerated.

The mixing of emulsions of different grain size or sensitometric characteristics for the purposes of altering the shape of the sensitometric curve produced from a step-wise image exposure of a photographic element made therefrom, is well known and is taught, for example, in British Patent Specification Nos. 428,305, May 10, 1935 and 732,691, June 29, 1955. It is also known to coat separate layers of different emulsions to achieve similar results as taught in U.S. Pat. No. 3,050,391, Aug. 21, 1962; U.S. Pat. No. 3,130,053, Apr. 21, 1964; U.S. Pat. No. 3,140,179, July 7, 1964; and in U.S. Pat. No. 3,418,123, Dec. 24, 1968. Such art, however, does not solve the problem of print-through when using high speed X'ray intensifying screens and double side coated films.

SUMMARY OF THE INVENTION It has been found that the problem of print-through can be greatly reduced while at the same time providing a high speed X-ray film with wide exposure latitude and good contrast. Such a film is provided by a film support having on both sides a low speed, fine grain silver halide emulsion layer adjacent to said support and a high speed silver halide emulsion layer coated on both of the low speed emulsion layers, the low speed emulsion layers having a silver halide grain size of 0.1 to 1.4 micron. The correct balance of grain size of the high and low speed emulsions is selected as taught in the prior art to achieve wide exposure latitude and good contrast. While the fast and slow emulsions can have the same grain size when the speed difference is effected solely by sensitization, it is preferred that the fast emulsion have larger grains than the slow emulsion.

The photographic speed of the high speed emulsion is from 2 to 10 times that of the low speed emulsion.

BRIEF DESCRIPTION OF THE DRAWING The accompanying drawing represents a cross section through a material of the present invention in enlarged scale wherein the outermost layer (1) is a nonphotosensitive anti-abrasion layer (preferred, but not necessary to the invention), (2) is a high speed emulsion layer, (3) is a low speed emulsion layer and (4) is the transparent support film. While the support film is represented as a single thickness, it may have subbing layers to improve the adhesion of the emulsion layers coated on it.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The low speed, fine grain emulsion used to prepare the layers coated adjacent to the transparent support is comprised of silver halide grains of about 0.1 micron to 1.4 micron size with 0.4 to 0. 8 micron being preferred. The high speed emulsion preferably has a larger silver halide grain size than the low speed emulsion, ranging from 1 to 2 microns and, more preferably, from 1.2 to 1.4 microns. The size of the high speed emulsion grains will depend on the grain size chosen for the low speed emulsion, since, as is well known, their ratio will affect the sensitometry of the film. By virtue of grain size and any sensitization, the high speed emulsion must have a photographic speed of from 2 to 10 times that of the low speed emulsion. The speed ratio of the emulsions is important since the slow emulsion must have a grain size small enough to be an effective light absorber and must also be slower to provide wide exposure latitude. On the other hand, it cannot be so slow (insensitive) that it is not effective in providing the desired sensitometric characteristics.

Preferably, the emulsions used in the layers of this invention are comprised mainly of silver bromide grains with small amounts (about 1 to 4 mole percent) of silver iodide. The emulsions are preferably prepared with a controlled amount of binder, so that the resulting product is processable in rapid access machines as taught, for example, by Barnes in U.S. Pat. No. 3,545,971. Preferably, the transparent film support used in this invention will also contain a blue dye incorporated therein as taught, for example, by Johnes et al., in British Patent Application No. 1,196,707, Oct. 28, 1970. A nonphotosensitive protective top coat of gelatin or other suitable material is preferably applied as a contiguous anti-abrasion layer on the high speed silver halide emulsion layers on each side of the support film.

Yet another embodiment of this invention involves the use of a silver chlorobromide emulsion for the preparation of the fine grain slower component layer coated on each side of the film base. These emulsion layers comprising about 50 mole percent or greater of silver chloride, and the remainder silver bromide, when used in conjunction with faster large grain silver bromide emulsion layers coated contiguous thereto, provide excellent results including wide latitude, reduced printthrough, high contrast and increased photographic speed. Additionally, processing advantages can also be achieved by using a high chloride emulsion since they are known to develop at a faster rate. Since the use of high chloride emulsions in the field of X-ray photography is not conventional, clue to low speed in compari- 3 son to bromide or bromoiodide emulsions, these results are indeed surprising.

The emulsions used in preparing the films of this invention may be made from any of the common silver halides (bromide, iodide or chloride) or mixtures of two or more of those mentioned. These emulsions are usually brought to their optimum sensitivity as regards speed, fog and contrast, with, for example, gold and sulfur as taught in US. Pat. Nos. 1,574,944; 1,623,499; 2,410,699; 2,399,083; 2,448,060 and 2,597,915 as well as the reducing agents of US. Pat. No. 2,487,850. Other sensitizing agents and speed increasing adjuvants well-known to those skilled in the art may also be advantageously employed herein. After sensitization is complete, other adjuvants (e.g. wetting and coating aids, hardeners, antifoggants and stabilizers, etc.) may also be added. The emulsions may be coated by conventional means (e.g., skim, air-knife, bar or falling film coating techniques) on any suitable planar support including glass, cellulose acetate, cellulose nitrate, and other synthetic film forming resins or polymers (e.g., polyesters polyamides, polystyrenes, etc.) They are preferably coated on a suitable, transparent film support (e.g., polyethylene terephthalate prepared and subbed as described in Alles US. Pat. No. 2,779,684, Example IV).

The emulsions of this invention may contain any of the well-known macromolecular organic colloids as binding agents such as gelatin, albumin, agar-agar, gum arabic, dextran, cellulose ether, partially hydrolysed cellulose acetate, polyvinyl alcohol, polyvinyl pyrrolidone, alkyl acrylate polymers and copolymers of mixed acrylates, the modified and hydrolysed gelatin of Rakoczy US Pat. No. 3,778,278, or mixtures of two or more of those described above may be used equally as well.

In practicingthe details of this invention one may prepare two separate emulsion components of inherent different emulsion speeds or one may prepare a single emulsion, split the emulsion into two portions and sensitize each portion so that one is no more than one-half the speed of the other. The slower portion is then coated adjacent to and on each side of a transparent support, while the faster portion is coated contiguous thereto, resulting in two layers on each side of the support. By adjusting the coating weights of the two layers and/or by providing a difference in grain size of the component emulsions one can also adjust the exposure latitude of the resulting product. This procedure is well-known in the art. The film elements of the invention have a considerable advantage in speed over comparable product made from mixed emulsions. This is important for medical radiological evaluations where a reduction in patient exposure to X-radiation is highly desirable. Such a multiple layer structure allows greater flexibility in that developer accelerators may be incorporated in the slower layer without producing deleterious side effects. Additionally, emulsion layers of different silver halide type, (e.g., chlorobromide in one layer and iodobromide in another) can be used. Mixed emulsions of such diverse emulsions would not produce equivalent results.

This invention will now be illustrated by the following examples:

EXAMPLE 1 Two separate emulsions were prepared. The first emulsion comprising silver iodobromide grains of about 0.7 to 08p. was about 98.5 mole percent silver bromide and about 1.5 mole percent silver iodide. The second emulsion comprising silver bromo-iodide grains of about 1.4 to 1.5,u in size had the same iodobromide ratio. Both emulsions were dispersed in gelatin (about 100 gm. gelatin per 1.5 moles silver halide) and brought to their optimum sensitivity by digestion with gold and sulfur at elevated temperatures (about 140F) by the conventional procedure. Because of the difference in grain size, however, the speed of emulsion l was only about 20% of the speed of emulsion 2. After digestion, the emulsions were cooled and the usual wetting and coating aids and antifoggers were added thereto. The emulsions were then coated on a 0.007 inch thick polyethylene terephthalate film base containing a blue dye which imparts a blue tint to the base, as described in British Pat. Application No. 1,196,707. The film base was first coated on both sides with a con ventional vinylidene chloride/alkyl acrylate/itaconic acid copolymer, over which had been coated on both sides a thin anchoring substratum of gelatin (about 0.5 mg/dm The coating system was set so as to coat emulsion l nearest to the base, emulsion 2 contiguous thereon and to simultaneously coat a gelatin antiabrasion layer (about 10 mg/dm on the top of emulsion 2. The coating conditions were set so that emulsion l was coated at about 15 mg silver halide/dm and emulsion 2 at about 60 mg silver halide/dm After drying, similar layers of identical composition and coating weight were applied to the other side of the film base so that the structure shown in the drawing was obtained wherein (1) is antiabrasion (10 mg gel/drn (2) is emulsion 2, (3) is emulsion 1 and (4) is the film base. A control element was also prepared by mixing emulsions 1 and 2 (20:80 parts respectively) and coating the mixed emulsion as a single layer on each side of the film base, so that the coating weight of the mixed emulsion was the same as the total coating weight of the combined layers described above. Samples of each one of these coatings were given an X-ray exposure at kVp, 20 ma for 2 sec. at 40 inches through a 2 aluminum step wedge, using a Bucky Grid, with two high speed medical X-ray screens (E. I. Du Pont de Nemours PAR Screens), one held in intimate contact with each side of the film element in a suitable cassette and the step wedge placed on top of the cassette. After exposure, the films were processed in a Kodak M-6 automatic processor at 33C, containing a standard p-N-methylamino phenol hydrosulfate/hydroquinone X-ray developer, with a total processing time of seconds (develop, fix, wash and dry). The reduction in print-through was deter mined by sensitometrically exposing a portion of film with only one screen and measuring the density of exposure at 0.5 above fog before and after the emulsion on the support side opposite to the side with the screen had been removed by washing. Thus, the difference in density (or as energy calculated to produce said density) is a measure of the amount of light from the X-ray screen that penetrated through the emulsion side nearest the screen and exposed the emulsion on the opposite side..The results are shown below:

Base Print Relative Top Print Through Sample Fog Speed Density Through Reduction(7 amma Mixed Emulsion (control) .28 100 2.72 3.30 25 Dual Layers Additionally, the image produced by the dual layer element of this invention appeared much sharper than that of the mixed emulsion control.

Both samples, when sensitometric curves were plotted, had the desired wide exposure latitude. The dual layer product of this invention had about a 12% in crease in speed and a 21% reduction in objectionable print-through. This is considered surprising, since it is a generally accepted conclusion that print-through in and of itself contributes higher density and thus higher apparent photographic speed.

EXAMPLE 2 emulsion 2 was as described in Example 1. The emulsions were coated as described previously with emulsion 1 having a final coating weight of 15 mg/dm and emulsion 2 having a final coating weight of 62 mg/dm A control element was also prepared without any TCA as well as a control single layer element from the mixed emulsions with the TCA in the emulsion and added to the abrasion overcoat. Samples from each of these coatings were exposed fresh and again after aging for 7 days in an oven at 52C and 65% relative humidity (which simulates long term aging). These samples were then processed as described in Example 1 and the following results were obtained:

Example 1 was repeated, but at a different coating weight wherein the layer adjacent to the film support (fine grain emulsion) was coated at 15 mg/dm A control sample containing both emulsions mixed and coated as a single layer was also made. Samples from these coatings were exposed and developed as described in Example 1 with the following results:

These experiments were repeated with other TCA derivatives (e. g., 2,2-dimethyl-L-thiazolidene-4-carboxylic acid; 2-pheny1-2-pentyl-L-thiazolidine-4-carboxylic acid; and 2,2-dibenzyl-L-thiazolidine-4-carboxylic acid) with similar results. This demonstrates that these developer accelerators can be more effectively utilized in this dual layer structure thus showing the remarkable Base Print- Relative Top Print- Through Sample Fog Speed Density Through Reduction(%) Mixed Emulsions (Control) .26 100 2.86 3.41 54 Dual Layers Both samples produced films with wide exposure latitude, but the film having the dual layers was not only sensitometrically improved as regards speed, but also gave a sharper image and reduced print-through.

EXAMPLE 3 In order to test the efficacy of the system of this invention when used with accelerators, e.g., development accelerators, the emulsions described in Example 1 were prepared. Emulsion l, the fine grain, slower emulsion. additionally contained 30 mg/l .5 moles of silver halide of L-thiazolidine'4carboxylic acid (TCA) while versatility of this system.

EXAMPLE 4 An emulsion comprising cubic silver chlorobromide grains of about 0.6g. was prepared having about mole percent silver chloride and about 30 mole percent silver bromide. This emulsion was then dispersed in gelatin so that the total gelatin was about g./1.5 moles of silver halide and was brought to its optimum sensitivity by digestion with gold and sulfur at about 128F as well-known to those skilled in the art. After this step, the emulsion was cooled and the usual wetting and coating aids, antifoggers and hardeners were added along with about 75 mg/l .5 moles of silver halide of 3,3'-diethylthiazoline carbocyanine dye (a blue sensitizing dye). The photographic speed of this emulsion was only about 40% of a second emulsion prepared similar to emulsion 2 described in Example 1 (i.e., 1.4-1.5;1. bromoiodide). Both emulsions were coated as dual layers on each side of a polyester film base as described in Example 1 at mg/dm and 85 mgldm respectively, the slow emulsion being coated next to the film base. A control was prepared by mixing and coating the same emulsions as a single layer. Samples from each coating were exposed and processed as described in Example 2 with the following results:

B Relative Sample Fog Speed Gradient Top Density Control- Mixed Emulsion .25 l()() 2.59 316 Dual Layers .2l I08 2.62 3.25

Additionally. although both samples exhibited a wide exposure latitude, the dual layer system showed reduced print-through and thus had a sharper image. 7

said low speed emulsion, and said low speed emulsion having a silver halide grain size of from 0.1 to 1.4 micron.

2. An X-ray film according to claim 1, said silver halide emulsions being gelatino emulsions.

3. An X-ray film according to claim 1, said high speed silver halide emulsion being a silver iodobromide emulsion and said low speed emulsion being a silver chlorobromide emulsion.

4. An X-ray film of claim 3, said low speed silver chlorobromide emulsion containing at least 50 mole percent chloride.

5. An X-ray film according to.claim 1, said high speed emulsion having a silver halide grain size of from i to 2 microns.

6. An X-ray film according to claim 1, said low speed emulsion having a silver halide grain size of 0.4 to 0.8 micron.

7. An X-ray film according to claim 1, said support being a transparent polymeric film.

8. An X-ray film according to claim 7, said support having gelatin subbing layers thereon.

9. An X-ray film according to claim 1, the high speed emulsion layers being overcoated with nonphotosensitive anti-abrasion layers.

10. A process of making improved radiographic images comprising exposing the X-ray film of claim 1 to Xradiation, there being X-ray intensifying screens on both sides of and in operative association with said film. 

1. AN IMPROVED X-RAY FILM COMPRISING A SUPPORT BEARING ON BOTH SIDES A LAYER OF A LOW SPEED, FINE GRAIN SILVER HALIDE EMULSION OVERCOATED WITH A LAYER OF A HIGH SPEED SILVER HALIDE EMULSION, THE HIGH SPEED EMULSION HAVING A PHOTOGRAPHIC SPEED FROM 2 TO 10 TIMES THAT OF SAID LOW SPEED EMULSION, AND SAID LOW SPEED EMULSION HAVING A SILVER HALIDE GRAIN SIZE OF FROM 0.1 TO 1.4 MICRON.
 2. An X-ray film according to claim 1, said silver halide emulsions being gelatino emulsions.
 3. An X-ray film according to claim 1, said high speed silver halide emulsion being a silver iodobromide emulsion and said low speed emulsion being a silver chlorobromide emulsion.
 4. An X-ray film of claim 3, said low speed silver chlorobromide emulsion containing at least 50 mole percent chloride.
 5. An X-ray film according to claim 1, said high speed emulsion having a silver halide grain size of from 1 to 2 microns.
 6. An X-ray film according to claim 1, said low speed emulsion having a silver halide grain size of 0.4 to 0.8 micron.
 7. An X-ray film according to claim 1, said support being a transparent polymeric film.
 8. An X-ray film according to claim 7, said support having gelatin subbing layers thereon.
 9. An X-ray film according to claim 1, the high speed emulsion layers being overcoated with nonphotosensitive anti-abrasion layers.
 10. A process of making improved radiographic images comprising exposing the X-ray film of claim 1 to X-radiation, there being X-ray intensifying screens on both sides of and in operative association with said film. 